Numbers: Allergies, From the Fecund Ragweed to Killer Peanuts

0.001 Fraction of a peanut, in powder form, given to children with severe peanut allergies at the start of a trial designed to build up their tolerance. Within a year, after gradual increases in the dosage, some of the youngsters safely consumed 15 peanuts before experiencing a reaction, according to allergist Wesley Burks of Duke University.

19,000 Maximum number of dust mites found in a gram of household dust (about the weight of a paper clip). One person in 10 is allergic to proteins in the bodies and waste of dust mites—tiny eight-legged creatures that multiply on surfaces such as bedding and carpeting.

50 Million Number of Americans who suffer from allergy-related ailments, including sinusitis, rhinitis (hay fever), and food allergies. Almost 55 percent of Americans experience a reaction to at least one allergen—the most common sources are trees, grasses, weeds, mold, and dust mites. Rhinitis alone affects 40 million people nationwide.

0.0000001 Grams of immunoglobulin E (IgE)—the germ-fighting antibody responsible for allergies—in an average liter of blood. Once the immune system detects an antigen such as pollen or pet dander, IgE triggers the release of histamine and other chemicals; these chemicals, produced by the body, are the actual cause of the familiar sneezing, itching, and watery eyes.

1 billion Approximate number of pollen grains produced by a single ragweed plant each year. As few as 10 grains per cubic meter of air can provoke symptoms in the most sensitive people; during late summer months the count can reach into the hundreds. Ragweed pollen triggers a reaction in about 75 percent of hay fever sufferers.